System Comprising an Integrated Waveguide-Coupled Optically Active Device and Method of Formation

ABSTRACT

Integrated-optics systems are presented in which an optically active device is optically coupled with a silicon waveguide via a passive compound-semiconductor waveguide. In a first region, the passive waveguide and the optically active device collectively define a composite waveguide structure, where the optically active device functions as the central ridge portion of a rib-waveguide structure. The optically active device is configured to control the vertical position of an optical mode in the composite waveguide along its length such that the optical mode is optically coupled into the passive waveguide with low loss. The passive waveguide and the silicon waveguide collectively define a vertical coupler in a second region, where the passive and silicon waveguides are configured to control the distribution of the optical mode along the length of the coupler, thereby enabling the entire mode to transition between the passive and silicon waveguides with low loss.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. Non-ProvisionalApplication Serial No. 17/484,370, filed Sep. 24, 2021, entitled “SystemComprising an Integrated Waveguide-Coupled Optically Active Device andMethod of Formation,” (Attorney Docket 3218-001US2), which is acontinuation-in-part of U.S. Non-Provisional Application Serial No.16/746,400 (now U.S. Pat. 11,131,806), filed Jan. 17, 2020, entitled“System Comprising an Integrated Waveguide-Coupled Optically ActiveDevice and Method of Formation,” (Attorney Docket 3218-001US1), whichclaims priority to U.S. Provisional Pat. Application Serial No:62/961,348, filed Jan. 15, 2020, entitled “System Comprising anIntegrated Waveguide-Coupled Optically Active Device and Method ofFormation,” (Attorney Docket 3218-001PR1), each of which is incorporatedby reference. If there are any contradictions or inconsistencies inlanguage between this application and one or more of the cases that havebeen incorporated by reference that might affect the interpretation ofthe claims in this case, the claims in this case should be interpretedto be consistent with the language in this case.

TECHNICAL FIELD

The present disclosure relates to heterogeneous integration ofcompound-semiconductor structures on silicon substrates in general, and,more particularly, to the integration of compound-semiconductor activeand passive photonic elements with silicon for forming semiconductorwaveguides, active electronic devices, and/or active and passivephotonic devices.

BACKGROUND

Silicon photonics promises relatively low-cost solutions for manyphotonic applications. However, historically, most silicon-photonicssystems cannot generate light on the chip. As a result, there areongoing efforts toward enabling on-chip light generation, amplification,modulation, etc. via heterogeneous silicon-photonic integration.

Some success in providing on-chip light generation, amplification,modulation, etc., in silicon-photonics systems has been achieved byintegrating compound-semiconductor material onto a substrate containingsilicon waveguides via direct bonding and subsequent formation of activecompound-semiconductor waveguides above silicon waveguides.

In some alternative prior-art approaches, discrete optically activedevices (e.g., lasers, amplifiers, modulators, etc.) are completelyformed separately and optically coupled with a photonic integratedcircuit (PIC) through, for example fiber coupling the device and thePIC, flip chip bonding the device onto the PIC substrate and opticallycoupling it with a silicon waveguide on the PIC via a silicon gratingcoupler, or employing conventional packaging methods wherein light iscoupled between the discrete device and a silicon waveguide on the PIC.

Unfortunately, the cost and complexity of such approaches have, thusfar, limited their use.

A platform that enables cost-effective formation of a heterogeneoussilicon photonic system in a cost-effective manner remains, as yet,unmet in the prior art.

SUMMARY

The present disclosure is directed toward integrated-optics systems thatinclude an optically active device optically coupled with a siliconwaveguide via a passive compound-semiconductor waveguide, where theoptically active device, the passive waveguide, and the siliconwaveguide reside on a common substrate. Embodiments in accordance withthe present disclosure are particularly well suited for use indistributed-feedback lasers, mode locked lasers, photonic integratedcircuits, external-cavity mode locked lasers, loop-mediated isothermalamplification devices, and the like.

An advance is made over the prior art by employing a passivecompound-semiconductor waveguide as a transition element between anoptically active device and a silicon waveguide. The optically activedevice includes active material that is disposed on and opticallycoupled with a compound semiconductor coupling waveguide. The activematerial resides completely in a first region of a substrate. Thesilicon waveguide is wholly contained in a second region of thesubstrate. The passive waveguide resides in a transition region betweenthe first and second regions and extends into these regions such thatthe passive waveguide is optically coupled with each of the opticallyactive device and silicon waveguide. The active material, the couplingwaveguide, the passive waveguide, and the silicon waveguide configuredto dictate the vertical location and lateral confinement of opticalenergy at each point along the length of the system.

Additional advances over the prior art are made by employing one or moreetch-stop layers at critical points in the fabrication process tofacilitate fabrication of narrow and/or sharp features (e.g., tapers) inone or more waveguide layers, where such features are extremelydifficult to form using conventional approaches. In some embodiments,narrow/sharp features are formed in a single etch process or,alternatively, via a two-step etching process in which a more-easilylithographically defined features is formed in a firstlithography/etching step, followed by a second etching step in whichcrystallographically dependent etch is used to further refine thefeature such that it is based on one or more sidewalls that are definedby a crystal plane of the etched material.

In addition, in some embodiments, an optical element, such as awaveguide grating, is included in the gain section of an opticallyactive device, where the optical element is formed in a silicon layer onwhich an active-material stack is disposed. In some such embodiments,the optically active device and optical element collectively define alaser structure, such as a distributed feedback laser.

The use of a compound-semiconductor passive waveguide as a transitionelement to optically couple an optically active device and a siliconwaveguide affords significant advantages over the prior art, such as:

-   i. independent control over the performance of the optically active    device and the coupling efficiency of a light signal into the    silicon waveguide, thereby enabling each to be substantially    optimized without degrading the other; or-   ii. transition of a light signal between the optically active device    and the silicon waveguide with less optical loss than can be    achieved in the prior art; or-   iii. in monolithically integrated systems, facilitated epitaxial    growth of the optically active device by virtue of the presence of a    compound semiconductor layer from which the passive waveguide is    formed in the first region;-   iv. in heterogeneously integrated systems, facilitated bonding of    nascent optical-device material on the common substrate by virtue of    the presence in the active region of a compound semiconductor layer,    from which the passive waveguide is formed; or-   v. any combination of i, ii, iii, and iv.

Further advance over the prior art is made by defining a couplingwaveguide and a passive waveguide from contiguous portions of acompound-semiconductor layer and forming a composite waveguide having arib portion that is an optically active device and a planar portion thatis the coupling waveguide. By tailoring the lateral dimensions of theoptically active device, the size and/or vertical position of an opticalmode of a light signal in the composite waveguide can be controlled. Asa result, the optical mode can be located at least partially in theoptically active device at one location and forced substantiallycompletely into the passive waveguide as the light signal propagates toa second location.

An illustrative embodiment is a waveguide-coupled optical system havingan active region, an output-coupling region, and a transition region,each of which is disposed on a silicon-on-insulator substrate. Thesystem includes: a quantum-dot laser whose active material is locatedcompletely within an active region, where it is disposed on andoptically coupled with a gallium arsenide coupling waveguide; a siliconwaveguide that resides completely in the output-coupling region; and apassive gallium-arsenide waveguide that resides in the transition regionand extends into each of the active and output-coupling regions where itis optically coupled with the laser and the silicon waveguide,respectively. As a result, the passive waveguide couples optical energygenerated by the laser into the silicon waveguide.

In the active region, the active material is patterned to define atapered region that defines a first coupler that facilitates transfer ofoptical energy generated in the laser structure into a light signalpropagating in the coupling waveguide.

In the output-coupling region, the silicon device layer of thesilicon-on-insulator substrate is patterned to define a siliconwaveguide that functions as a single-mode waveguide for the lightsignal.

In the transition region, the coupling waveguide is patterned to definea single-mode passive waveguide for the light signal. No opticallyactive material or electrical contacts are present in the transitionregion. The passive waveguide extends slightly into the output-couplingregion, where it is tapered to define a second coupler that facilitatestransfer of the light signal into the silicon waveguide.

In some embodiments, the coupling layer includes a lower sub-layer andan upper sub-layer, where the lower sub-layer is doped to facilitateforming electrical contacts. The upper sub-layer is characterized by ahigher refractive index than the lower sub-layer, thereby enabling thelower sub-layer to also function as a lower cladding layer. In someembodiments, the upper sub-layer is characterized by a lower refractiveindex than the lower sub-layer.

In some embodiments, the optically active device is a device other thana laser, such as an optical modulator (e.g., an electroabsorptionmodulator, a phase modulator, etc.), an optical amplifier, a variableoptical attenuator, a photodetector, and the like. In some embodiments,the optically active device includes a quantum element other than aquantum dot, such as a quantum dash, quantum well, a quantum wire, andthe like. In some embodiments, optically active device does not includea quantum element.

In some embodiments, the active region includes an optically activedevice and coupling layer of a different compound semiconductor, such asindium phosphide, indium gallium arsenide phosphide, and the like. As aresult, the passive waveguide also comprises this different compoundsemiconductor.

In some embodiments, a reflector is defined in at least one of thepassive and silicon waveguides to redirect a light signal. In someembodiments, the reflector is defined in the passive waveguide and isconfigured to optically couple with a vertical grating coupler definedin the silicon waveguide.

In some embodiments, an alignment feature is included for passivelyaligning a bulk optical element (e.g., an optical fiber, aphotodetector, a light source, etc.), to one of the passive and siliconwaveguides. In some such embodiments, this alignment feature is asilicon-optical-bench feature.

In some embodiments, the silicon waveguide is not included and thepassive waveguide functions as an optical interface to another opticalelement. In some embodiments, the other optical element and theoptically active device are disposed on the same substrate. In someembodiments, the other optical element is external to the substratecomprising the optically active device. In some such embodiments, thepassive waveguide is configured such that its optical mode issubstantially matched with the optical mode of an external element tomitigate optical coupling loss.

In some embodiments, a spot-size converter is included in the passivewaveguide and/or the silicon waveguide to facilitate optical couplingwith an external element.

An embodiment in accordance with the present disclosure is anintegrated-optics system disposed on a substrate that defines a firstplane, the system comprising: (1) an optically active device that isselectively located in a first region of the substrate, wherein theoptically active device includes: (a) an active-material stackcomprising a gain section and a first taper; and (b) a couplingwaveguide that includes a first layer disposed on a contact layer, thefirst layer comprising a first compound semiconductor and the contactlayer comprising a second compound semiconductor, wherein the couplingwaveguide at least partially supports a first optical mode of a lightsignal; wherein the active-material stack and the coupling waveguidecollectively define a composite waveguide that at least partiallysupports the first optical mode; and (2) a silicon layer that includes asilicon waveguide that is selectively located in a second region of thesubstrate, the silicon waveguide being configured to at least partiallysupport the first optical mode; and wherein the coupling waveguideextends from the first region to a second taper located in the secondregion, the second taper being configured to optically couple the firstoptical mode from the composite waveguide into the silicon waveguide.

Another embodiment in accordance with the present disclosure is a methodfor forming an integrated-optics system disposed on a first substratethat defines a first plane, the system having a first region, a secondregion, and a third region, and comprising an optically active devicethat is optically coupled with a silicon waveguide, the methodincluding: forming a coupling waveguide that extends from the firstregion to the third region, the coupling waveguide including a firstlayer disposed on a contact layer, the first layer comprising a firstcompound semiconductor and the contact layer comprising a secondcompound semiconductor, wherein the coupling waveguide at leastpartially supports a first optical mode of a light signal; forming anoptically active device that includes an active-material stackcomprising a third compound semiconductor, the active-material stackbeing disposed on the coupling waveguide to collectively define acomposite waveguide that at least partially supports the first opticalmode, wherein the optically active device is located only in the firstregion; and patterning a silicon layer to define a silicon waveguide inthe third region, the silicon waveguide being configured to at leastpartially support the optical mode, wherein the second waveguide isoptically coupled with the active-material stack via the couplingwaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of a top view of an illustrativeintegrated-optics system in accordance with the present disclosure.

FIGS. 2A-E depict sectional views of system 100 through lines a-athrough e-e, respectively.

FIG. 3 depicts operations of a method suitable for forming system 100 inaccordance with the illustrative embodiment.

FIG. 4A depicts a cross-sectional drawing of a portion of a sacrificialsubstrate comprising coupling layer 208 and active-material stack 210.

FIG. 4B depicts a schematic drawing of a cross-sectional view of nascentsystem 100' after the removal of sacrificial substrate 402.

FIG. 5A depicts a schematic drawing of a sectional view of analternative composite waveguide in accordance with the presentdisclosure.

FIG. 5B depicts a schematic drawing of a sectional view of anotheralternative composite waveguide in accordance with the presentdisclosure.

FIG. 6 depicts a schematic drawing of a sectional view of an alternativetransition region in accordance with the present disclosure.

FIG. 7 depicts a schematic drawing of a sectional view of an alternativeembodiment of a coupler for optically coupling a passive waveguide and asilicon waveguide in accordance with the present disclosure.

FIG. 8 depicts a schematic drawing of a cross-sectional view of yetanother alternative coupler in accordance with the present disclosure.

FIGS. 9A-B depict schematic drawings of cross-sectional views ofalternative output ports in accordance with the present disclosure.

FIG. 10 depicts a schematic drawing of an alternative system inaccordance with the present disclosure.

FIG. 11 depicts a schematic drawing of a cross-sectional view of anotheralternative system in accordance with the present disclosure.

FIG. 12 depicts a schematic drawing of a top view of yet anotheralternative embodiment of an integrated-optics system in accordance withthe present disclosure.

FIGS. 13A-E depict schematic drawings of sectional views of system 1200through lines f-f through j-j, respectively.

FIG. 14 depicts operations of a method suitable for forming system 1200in accordance with the present disclosure.

DETAILED DESCRIPTION

For the purposes of the present disclosure, including the appendedclaims, the following terms are defined:

-   “disposed on” or “formed on” is defined as is defined as “exists on”    an underlying material or layer with or without intermediate layers.    For example, if a material is described to be “disposed (or grown)    on a substrate,” this can mean that either (1) the material is in    intimate contact with the substrate; or (2) the material is in    contact with one or more layers that reside on the substrate.-   optically active device is defined as an electrically coupled device    (i.e., a device comprising electrical contacts for connecting to    external circuitry) in which, in response to an electrical signal    applied to the electrical contacts, either (a) photons are generated    due to the recombination of free carriers;(b) free-carrier pairs are    generated due to the absorption of photons; or (c) the phase of one    or more photons is modified by an injected current or voltage.    Examples of optically active devices include, without limitation,    lasers, optical amplifiers, optical modulators (e.g.,    electroabsorption modulators, phase modulators, etc.), variable    optical attenuators, photodetectors, and the like. It should be    noted that an optically active device does not require inclusion of    a quantum-element-containing layer.-   quantum element is defined as a semiconductor structure that    exhibits a quantum effect. Examples of quantum elements include,    without limitation, quantum dots, quantum wells, quantum-well    layers, quantum dashes, quantum wires, and the like.-   passive waveguide is defined as a surface waveguide in which light    passes through virtually unperturbed. Passive waveguides are not    operatively coupled with electrical contacts and are not stimulated    to exhibit optoelectronic effects, such conversion of free carriers    into photons or vice versa, optical modulation, optical    amplification, and the like.

For the purposes of this Specification, including the appended claims,the terms “lateral” and “vertical” as used are meant to be relative tothe major surfaces of a substrate on which an integrated-optics systemresides, where the term lateral refers to directions that are parallelto the major surfaces and the term vertical refers to directions thatare normal to the major surfaces. In similar fashion, the term “lower”means more proximate to the substrate and the term “higher” means moredistal from the substrate.

FIG. 1 depicts a schematic drawing of a top view of an illustrativeintegrated-optics system in accordance with the present disclosure.System 100 includes active region 102, transition region 104, andoutput-coupling region 106, which are disposed on substrate 108. Activeregion 102, transition region 104, and output-coupling region 106 arecontiguous regions that collectively define an integrated-optics-based,silicon-waveguide-coupled laser. System 100 comprises optically active(OA) device 110, coupling waveguide 112, passive waveguide 114, andsilicon waveguide 116, which are arranged such that optical energygenerated by OA device 110 propagates as light signal 118 from the laserto port 120.

It should be noted that, although the illustrative embodiment is anintegrated-optics-based, silicon-waveguide-coupled laser wherein lightis generated in an optically active device and propagates to a waveguideport, embodiments in accordance with the present disclosure includesystems wherein a light signal is coupled into a waveguide port andconveyed to an OA device. Furthermore, in some embodiments, no siliconwaveguide is included and port 120 is located in passive waveguide 114.Furthermore, in some embodiments, more than one passive waveguide and/orsilicon waveguide is included.

FIGS. 2A-E depict schematic drawings of sectional views of system 100through lines a-a through e-e, respectively.

FIG. 3 depicts operations of a method suitable for forming system 100 inaccordance with the illustrative embodiment. Method 300 is describedwith continuing reference to FIGS. 1 and 2A-E. Method 300 begins withoperation 301, wherein silicon waveguide 116 is defined on substrate 108in output-coupling region 106.

Substrate 108 is a conventional silicon-on-insulator (SOI) substratecomprising handle substrate 202, buried oxide layer (BOX) 204, andsilicon device layer 206. In the depicted example, handle substrate 202is a conventional silicon wafer, BOX 204 is a layer of thermally grownsilicon dioxide having a thickness that is typically within the range ofapproximately 1-2 microns and, preferably, 2 microns, and silicon devicelayer 206 is a layer of single-crystal silicon having a thickness equalto approximately 220 nm. As will be apparent to one skilled in the artafter reading this Specification, however, substrate 108 can be anysubstrate suitable for use in system 100. Examples of substratessuitable for use in accordance with the present disclosure include,without limitation, glass substrates, compound-semiconductor substrates,bulk silicon substrates, and the like.

Silicon waveguide 116 is a rib waveguide having a rib portion of widthw6. In output-coupling region 106, silicon waveguide is configuredenable the waveguide to support single-mode propagation of light signal118. Silicon waveguide 116 is formed by patterning silicon device layer206 via conventional lithography and etching to define its structure. Insome embodiments, silicon waveguide 116 has a waveguide structure otherthan that of a rib waveguide, such as a channel waveguide, stripwaveguide, ridge waveguide, etc.

At operation 302, coupling layer 208 and active-material stack 210 areadded to substrate 108.

In the depicted example, heterogeneous integration techniques are usedto add coupling layer 208 and active-material stack 210 to substrate108. Heterogeneous integration techniques suitable for use in accordancewith the present disclosure are described in, for example, U.S. Pat’s9,097,848, 9,910,120, 8,830,033, 8,620,164, each of which isincorporated herein by reference in their entirety.

In accordance with conventional heterogeneous integration, a separatephotonic substrate is formed by epitaxially growing active-materialstack 210 and coupling layer 208 on a sacrificial substrate.

FIG. 4A depicts a schematic drawing of a cross-sectional view of aportion of a photonic substrate in accordance with the illustrativeembodiment. Photonic substrate 400 includes coupling layer 208 disposedon active-material stack 210, which is disposed on sacrificial substrate402.

In the depicted example, sacrificial substrate 402 is a conventionalgallium arsenide wafer and coupling layer 208 is a layer of galliumarsenide having a thickness suitable for supporting single-modepropagation of light signal 118. In some embodiments, at least one ofsacrificial substrate 402 and coupling layer 208 comprises a compoundsemiconductor other than gallium arsenide, such as indium phosphide,indium gallium arsenide, indium gallium arsenide phosphide, and thelike.

Preferably, coupling layer 208 comprises sub-layers 208A and 208B, wheresub-layer 208A has a lower refractive index than that of sub-layer 208B.As a result, sub-layer 208A can function as a cladding layer that servesto substantially confine at least a portion of the optical mode of lightsignal 118 to sub-layer 208B as the light signal propagates throughpassive waveguide 114. In some embodiments, coupling layer 208 includesat least one additional sub-layer distal to sub-layer 208A, where thisadditional sub-layer or sub-layers are configured to function as anupper cladding for sub-layer 208B. In some embodiments, coupling layer208 does not include sub-layers (i.e., it is a homogeneous layer).

Furthermore, in some embodiments, sub-layer 208A is doped to reducecontact resistance for contacts 122 n.

Active-material stack 210 includes the constituent layers of OA device110, including cladding layers, carrier confinement layers, and gainlayer 212. In the depicted example, OA device 110 is a quantum-dot lasercomprising active-material stack 210. In some embodiments, OA device 110is a different optically active device, such as an optical modulator(e.g., an electroabsorption modulator, a phase modulator, etc.), anoptical amplifier, a variable optical attenuator, a photodetector, andthe like.

It should be noted that, although the illustrative embodiment includes again layer comprising a plurality of quantum dots, gain layer 212 caninclude one or more layers comprising any one or more of a wide varietyof quantum elements without departing from the scope of the presentdisclosure. Quantum elements suitable for inclusion in gain layer 212include, without limitation, quantum wells, quantum-well layers, quantumwires, quantum dashes, and the like.

In addition, as discussed below, preferably, active-material stack 210also includes sub-layers having different refractive indices to controlthe vertical position of optical mode 216. For example, in someembodiments, active-material stack 210 contains higher aluminum contentwith gallium arsenide in the layers between gain layer 212 and its topcontact layer. This provides a lower index of refraction that can forceoptical energy in an optical mode within the material stack downwardtoward coupling waveguide 112. Furthermore, defining the active-materialstack as a ridge or rib also helps force the optical mode toward thecoupling waveguide while serving to laterally contain the optical modeas well.

In some embodiments, a dielectric layer is included between couplinglayer 208 and silicon device layer 206 to act as a lower cladding thatconfines the optical mode toward at least one of the middle portion ofpassive waveguide 114 and the middle portion of active-material stack210.

In some embodiments, active-material stack 210 also includes a regionbetween gain layer 212 and coupling waveguide 112 that has higheraluminum content to create a lower index of refraction, thereby forcingthe optical mode upward away from the coupling waveguide. It should benoted that this same layer can also function as an upper cladding forcoupling waveguide 112.

Once active-material stack 210 and coupling layer 208 are complete,photonic substrate 400 is then flipped over and coupling layer 208 isbonded to silicon device layer 206 via direct bonding. In someembodiments, a different bonding technology is used to join couplinglayer 208 and device layer 206, such as plasma bonding, fusion bonding,thermo-anodic bonding, and the like. In some embodiments, an interfacelayer is included between silicon device layer 206 and coupling layer208 to facilitate their bonding.

Once photonic substrate 400 and substrate 108 are bonded, sacrificialsubstrate 402 is removed in conventional fashion.

FIG. 4B depicts a schematic drawing of a cross-sectional view of nascentsystem 100' after the removal of sacrificial substrate 402.

Although in the depicted example, coupling layer 208 and active-materialstack 210 are added to substrate 108 via heterogeneous bondingtechniques, in some embodiments, they are epitaxially grown on thesubstrate. In such embodiments, coupling layer 208 is grown directly onsilicon device layer 206 via hetero-epitaxial growth, which is followedby epitaxial growth of active-material stack 210 on the coupling layer.

Returning now to method 300, at operation 303, active-material stack 210is patterned to define the lateral dimensions of OA device 110. Itshould be noted that this removes active material completely from eachof transition region 104 and output-coupling region 106. As a result,optically active material is selectively included in active region 102.However, as will be apparent to one skilled in the art, after readingthis Specification, in some embodiments, a substrate has multiple activeregions, each containing corresponding patterns of active-material stack210. In some embodiments, at least one of the constituent layers ofactive-material stack 210 is patterned using a mask specific to thatlayer, while at least one other of the constituent layers is patternedusing a different mask.

OA device 110 is patterned such that it has nominal width w1 outside ofthe area of coupler 128-1, where the value of w1 is selected tofacilitate lateral confinement of light signal 118 in the active region.In the depicted example, w1 is equal to 2 microns; however, w1 can haveany suitable value. Typically, w1 is within the range of approximately0.5 micron to approximately 4 microns.

At operation 304, coupling layer 208 is patterned in conventionalfashion to define coupling waveguide 112 in active region 102 andpassive waveguide 114 in transition region 104, where the passivewaveguide supports single-mode propagation of light signal 118. Itshould be noted, however, that in active region 102, coupling waveguide112 is not typically a single-mode waveguide for light signal 118.

Coupling waveguide 112 is formed as a rib waveguide. Coupling waveguide112 includes a central ridge portion having width w2 and a planarportion having width w3. In the depicted example, w2 and w3 are 6microns and 100 microns, respectively; however, each of w2 and w3 canhave any suitable value. Typically, the value of w2 is within the rangeof approximately 0.5 micron to 20 microns. As will be apparent to oneskilled in the art, the value of w3 is not typically critical, but isnormally within the range of 25 to 500 microns.

It should be noted that, after operation 304, active-material stack 210and coupling waveguide 112 collectively define composite waveguide 214as a rib waveguide (also sometimes referred to as a strip waveguide),where the active-material stack functions as the projecting ridgeportion of the composite waveguide and the central ridge portion ofcoupling waveguide 112 functions as the planar portion of the ribwaveguide.

In the depicted example, the width, w2, of coupling waveguide 112 isselected such that the coupling waveguide, itself, contributes little orno lateral confinement of optical mode 216 within active region 102. Asa result, the shape and size of optical mode 216 in active region 102 isdetermined primarily by the lateral dimensions of active-material stack210. In some embodiments, however, coupling waveguide 112 has a widththat enables the coupling waveguide to provide lateral confinement ofthe optical mode. In some embodiments, the width of coupling waveguide112 is substantially equal to the width of active-material stack 210(i.e., w2=w1 and the coupling waveguide is substantially a channelwaveguide).

Passive waveguide 114 is also formed as a rib waveguide comprising acentral ridge portion having width w4 and a planar region having widthw5. In the depicted example, w4 and w5 are 2 microns and 10 microns,respectively; however, each of w4 and w5 can have any suitable value.Typically, the value of w4 is within the range of approximately 0.5micron to 4 microns. As will be apparent to one skilled in the art, thevalue of w5 is not typically critical, but is normally within the rangeof 0.5 to 100 microns.

Since coupling waveguide 112 and passive waveguide 114 are continuoussegments of coupling layer 208, they are inherently optically coupled.

At operation 304, contacts 122 n are formed outside the central ridgeportion of coupling waveguide 112.

At operation 305, contact 122 p is formed on the top surface of OAdevice 110 to complete the formation of system 100. It should be notedthat, in some embodiments, the doping profile of the III-V materiallayers of gain section 1210 is reversed and, as a result, the positionsof contacts 122 n and 122 p are reversed.

It should be noted that, in some embodiments, coupling waveguide is aslab waveguide (i.e., no ridge and planar portions are defined incoupling layer 208 within active region 102). In such embodiments,contacts 122 n can be formed on the top surface of the coupling layer,in vias partially etched down to sub-layer 208A, or in any mannersuitable for making them operatively coupled with OA device 110. In someembodiments, coupling waveguide 112 is formed as a channel waveguide(i.e., no planar portion remains after coupling layer 208 has beenetched to define the waveguide). In such embodiments, device layer is inelectrical contact with sub-layer 208A and contacts 122 n are formed insilicon device layer 206.

Upon completion of system 100, active region 102 selectively includes OAdevice 110 and coupling waveguide 112, output-coupling region 106selectively includes silicon waveguide 116, and transition region 104includes passive waveguide 114. Passive waveguide 114 also extends intoactive region 102 to form coupler 128-1 with coupling waveguide 112 andextends into output-coupling region 106 to form coupler 128-2 withsilicon waveguide 116.

It is an aspect of the present invention that the lateral dimensions ofactive-material stack 210 substantially determine the vertical positionat which optical energy in OA device 110 forms an optical mode, as wellas the shape of that optical mode. As a result, active-material stack210 is defined such that it includes a first segment (i.e., gain section124) that is configured to favor optical gain that gives rise to anoptical mode and a second segment (i.e., taper 126-1) that is configuredto force that optical mode into coupling waveguide 112.

FIG. 2A depicts a schematic drawing of a sectional view of OA device 110taken through gain section 124 (i.e., through line a-a shown in FIG. 1).

As seen in FIG. 2A, in gain section 124, the relative values of w1 andw2 give rise to optical mode 216 such that its optical energy iscontained within one continuous region that is substantially centrallylocated in composite waveguide 214. In other words, each of OA device110 and coupling waveguide 112 partially supports optical mode 216.

In some embodiments, coupling waveguide 112 and active-material stack210 are configured such that optical mode 216 extends over a continuousregion that includes at least a portion of each of the active-materialstack, the coupling waveguide, and silicon device layer 206 (i.e.,optical mode 216 is partially supported by each of the active-materialstack, the coupling waveguide, and the silicon device layer).

Coupler 128-1 is a section of active region 102 configured for forcingsubstantially all of the optical energy of light signal 118 located inactive-material stack 210 into coupling waveguide 112 so that it canefficiently couple into passive waveguide 114. Coupler 128-1 includestapers 126-1 and 126-2.

Taper 126-1 is a segment of active-material stack 210 that is configuredto force the optical energy of light signal 118 into coupling waveguide112. Taper 126-1 has length L1 and a width that reduces from w1 to zero(i.e., extinction) along length L1. In the depicted example, L1 is equalto 100 microns; however, it is typically within the range ofapproximately 50 microns to approximately 500 microns. It should benoted that the value of L1 is a matter of design choice and, therefore,it can have any suitable value. Furthermore, in some embodiments, taper126-1 does not taper to extinction but, rather, to a non-zero width(e.g., one micron or less) that is sufficiently narrow to force theoptical energy of light signal 118 from OA device 110 into passivewaveguide 114.

In similar fashion, taper 126-2 is a segment of coupling waveguide 112that is configured to facilitate the transfer of light signal 118 intopassive waveguide 114 as a single-mode signal. Taper 126-2 has lengthL2, over which the width of coupling waveguide 112 changes from w2 to w4(i.e., the width of passive waveguide 114). In the depicted example, L2is equal to 50 microns; however, it is typically within the range ofapproximately 10 microns to approximately 500 microns. It should benoted that the value of L2 is not critical and, therefore, it can haveany value within a wide range.

FIG. 2B depicts a schematic drawing of a sectional view of coupler 128-1as taken through line b-b shown in FIG. 1 .

As noted above, the distribution of the optical energy of optical mode216 between active-material stack 210 and coupling layer 208 (i.e., theshape and vertical position of the optical mode) is based on therelationship of w1 and w2, which change along length L1 of taper 126-1.Tapers 126-1 and 126-2 are configured, therefore, to force optical mode216 substantially completely into coupling waveguide 112 by the timelight signal 118 reaches transition region 104.

In similar fashion, in the depicted example, w4 and w5 of passivewaveguide 114 are selected such that optical mode 216 is substantiallyconfined within its ridge and planar regions within transition region104. In some embodiments, however, passive waveguide 114 is configuredsuch that the optical energy of optical mode 216 extends across acontinuous region that occupies at least portions of both passivewaveguide 114 and silicon device layer 206. In other words, each ofpassive waveguide 114 and silicon device layer 206 partially supportsoptical mode 216.

FIG. 2C depicts a schematic drawing of a sectional view of transitionregion 104 as taken through line c-c shown in FIG. 1 .

Coupler 128-2 is a section of output-coupling region 106 configured forefficiently optically coupling light signal 118 from passive waveguide114 into silicon waveguide 116. Coupler 128-2 includes taper 126-3 and asegment of silicon waveguide 116, where taper 126-3 is a segment ofpassive waveguide 114 that is configured to facilitate the transfer oflight signal 118 into silicon waveguide 116 as a single-mode signal.

FIG. 2D depicts a schematic drawing of a sectional view of coupler 128-2as taken through line d-d shown in FIG. 1 . Note that the planar portionof silicon waveguide 116 is not shown in FIG. 1 .

Taper 126-3 has length L3, over which the width of passive waveguide 114changes from w4 to extinction. As a result, taper 126-3 is configured toforce optical mode 216 completely into silicon waveguide 116. In thedepicted example, L3 is equal to 200 microns; however, it is typicallywithin the range of approximately 50 microns to approximately 1000microns. It should be noted that the value of L3 is a matter of designchoice and, therefore, it can have any suitable value. As discussedabove and with respect to taper 126-1, in some embodiments, taper 126-3does not taper to extinction but, rather, to a non-zero width (e.g., onemicron or less) that is sufficiently narrow to force the optical energyof light signal 118 from passive waveguide 114 into silicon waveguide116. Furthermore, in some embodiments, silicon waveguide 116 includes ataper that facilitates transfer of optical energy between the siliconand passive waveguides. In some embodiments, both passive waveguide 114and silicon waveguide 116 include a taper.

FIG. 2E depicts a schematic drawing of a sectional view of siliconwaveguide 116 as taken through line e-e shown in FIG. 1 . It should benoted that, in the depicted example, the regions between siliconwaveguide 116 and taper 126-3 are air-cladding regions; however, theseregions can be filled with any material suitable to function as claddingmaterial for the silicon waveguide, such as silicon dioxide, siliconnitride, polymer, and the like.

It is another aspect of the present invention that OA device 110 can beconfigured to support an optical mode that is discontinuous such that,at some points within active region 102, it includes separateoptical-mode portions that propagate together but are distributed amongactive-material stack 210 and coupling waveguide 114 and, in someembodiments, silicon device layer 206. In other words, each ofactive-material stack 210 and coupling waveguide 114 and silicon devicelayer 206 partially supports optical mode 216 by supporting a differentone of its optical-mode portions.

FIG. 5A depicts a schematic drawing of a sectional view of analternative composite waveguide in accordance with the presentdisclosure. The sectional view depicted in FIG. 5A is taken through aregion analogous to that intersected by line a-a shown in FIG. 1 .Composite waveguide 500 includes substrate 108, coupling waveguide 502,and active-material stack 504. For clarity, electrical contacts are notshown in FIG. 5A.

In composite waveguide 500, coupling waveguide 502 and active-materialstack 504 are configured such that each includes at least one sub-layerthat is configured to force optical energy of optical mode 216 into adifferent sub-layer of that element. As a result, optical mode 216 issplit into two discontinuous optical-mode portions - optical-modeportions 216A and 216B. In other words, each of coupling waveguide 502and active-material stack 504 partially supports optical mode 216 bysupporting a different one of optical-mode portions 216A and 216B.

Coupling waveguide 502 is analogous to coupling waveguide 112; however,coupling waveguide 502 has width w 8 and includes sub-layers 502A, 502B,and 502C, where each of sub-layers 502A and 502C has a refractive indexthat is lower than that of sub-layer 502B. As a result, sub-layers 502Aand 502C function as lower and upper cladding layers, respectively, thatsubstantially confine the bulk of the optical energy of optical-modeportion 216A to sub-layer 502B.

Active-material stack 504 is analogous to active-material stack 210;however, active-material stack 504 has width w7 and includes sub-layers506A and 506B, each of which has a refractive index that is higher thanthat of gain layer 212. As a result, sub-layers 506A and 506B functionas upper and lower cladding layers, respectively, that substantiallyconfine the bulk of the optical energy of optical-mode portion 216B tothe portion of active-material stack 504 that resides between them.

The widths of optical-mode portions 216A and 216B and the spacingbetween them are based upon widths w7 and w 8 and the sub-layerconfigurations of coupling waveguide 502 and active-material stack 504.

In some embodiments, coupling waveguide 502 and active-material stack504 are configured such that optical mode 216 includes a thirdoptical-mode portion that is located in silicon device layer 206.

FIG. 5B depicts a schematic drawing of a sectional view of anotheralternative composite waveguide in accordance with the presentdisclosure. The sectional view depicted in FIG. 5B is taken through aregion analogous to that intersected by line a-a shown in FIG. 1 .Composite waveguide 508 is analogous to composite waveguide 500;however, in composite waveguide 508, coupling waveguide 502 andactive-material stack 504 are configured to split the optical energy ofoptical mode 216 into three discontinuous optical-mode portions -optical-mode portions 216C, 216D, and 216E.

Coupling waveguide 510 is analogous to coupling waveguide 502; however,coupling waveguide 510 has width w 10 and includes sub-layers 502D,502E, and 502F, where each of sub-layers 502D and 502F has a refractiveindex that is lower than that of sub-layer 502E. As a result, sub-layers502D and 502F function as lower and upper cladding layers, respectively,that substantially confine the bulk of the optical energy ofoptical-mode portion 216D to sub-layer 502E.

Active-material stack 512 is analogous to active-material stack 504;however, active-material stack 512 has width w 9 and includes sub-layers514A and 514B, each of which has a refractive index that is lower thanthat of gain layer 212. As a result, sub-layers 514A and 514B functionas upper and lower cladding layers, respectively, that substantiallyconfine the bulk of the optical energy of optical-mode portion 216C tothe portion of active-material stack 512 that resides between them.

In the depicted example, coupling waveguide 510 and active-materialstack 512 are further configured to give rise to additional optical-modeportion 216E, which is discontinuous with optical-mode portions 216C and216D and substantially confined to silicon device layer 206.

As a result, each of coupling waveguide 510, active-material stack 512,and silicon device layer 206 partially supports optical mode 216 bysupporting a different one of its optical-mode portions. The widths ofoptical-mode portions 216C, 216D, and 216E, as well as the spacingbetween them, are based upon widths w 9 and w 10 and the sub-layerconfigurations of coupling waveguide 510 and active-material stack 512.

In some embodiments, coupling layer 208 is configured such that opticalmode 216 is split into discontinuous optical-mode portions in transitionregion 104, with one of the optical-mode portions being located inpassive waveguide 112 and another optical-mode portion is located insilicon device layer 206.

FIG. 6 depicts a schematic drawing of a sectional view of an alternativetransition region in accordance with the present disclosure. Thesectional view depicted in FIG. 6 is taken through a region analogous tothat intersected by line c-c shown in FIG. 1 . Transition region 600includes substrate 108, passive waveguide 602, and cladding layer 604.Transition region 600 is analogous to transition region 104 describedabove; however, in transition region 600, optical mode 216 includesdiscontinuous optical-mode portions 216E and 216F, which reside inpassive waveguide 602 and silicon device layer 206, respectively.

Passive waveguide 602 is analogous to passive waveguide 114; however,passive waveguide 602 includes sub-layer 606, which has a refractiveindex that is higher than the remainder of the passive waveguide.

Cladding 604 is a thin layer of material suitable for substantiallyblocking the passage of optical energy of light signal 118 betweenpassive waveguide 114 and silicon device layer 206. In the depictedexample, cladding 604 is a layer of silicon dioxide having a thicknessof approximately 100 nm; however, it will be clear to one skilled in theart, after reading this Specification, how to specify, make, and usealternative embodiments wherein cladding 604 comprises a differentmaterial and/or has a different thickness.

As noted above, passive waveguide 602, cladding 604 and sub-layer 606are configured such that they collectively support optical mode 216,which includes discontinuous optical-mode portions 216F and 216G, whichare located in passive waveguide 114 and silicon device layer 206,respectively (i.e., passive waveguide 114 partially supports opticalmode 216 by supporting optical-mode portion 216F and silicon devicelayer 206 partially supports optical mode 216 by supporting optical-modeportion 216G). The shapes, vertical positions, and separation betweenoptical-mode portions 216F and 216G are based on the values of w11 thatlocation.

FIG. 7 depicts a schematic drawing of a sectional view of an alternativeembodiment of a coupler for optically coupling a passive waveguide and asilicon waveguide in accordance with the present disclosure. Thesectional view depicted in FIG. 7 is taken through a region analogous tothat intersected by line d-d shown in FIG. 1 . Coupler 700 includessubstrate 108, silicon waveguide 116, cladding 702, and taper 704.

Cladding 702 is analogous to cladding 604.

Taper 704 is analogous to taper 126-3; however, taper 704 has widthw12(x) along length L3 and includes sub-layer 706, which has arefractive index that is higher than the remainder of the taper. As aresult, sub-layer 706 serves to confine optical energy of light signal118 to the region of taper 704 located between cladding 702 andsub-layer 706.

By virtue of cladding 702 and sub-layer 706, optical mode 216 includesdiscontinuous optical-mode portions 216H and 216J, which are located inand supported by silicon waveguide 116 and taper 704, respectively. As aresult, each of silicon waveguide 116 and taper 704 partially supportsoptical mode 216. The shapes, vertical positions, and separation betweenoptical-mode portions 216H and 216J at any location along length L3 arebased on the values of w12(x) and w6 at that location and theconfiguration of taper 704 and sub-layer 706.

In some embodiments, light signal 118 is optically coupled betweenpassive waveguide 114 and silicon waveguide 116 via a turning reflectorand vertical grating coupler.

FIG. 8 depicts a schematic drawing of a cross-sectional view of yetanother alternative coupler in accordance with the present disclosure.Coupler 800 includes reflector 802 and vertical grating coupler 804.

Reflector 802 is an angled facet formed in passive waveguide 114.Reflector 802 is configured to receive light signal 118 propagatingalong longitudinal axis A1 of the passive waveguide and redirect italong a direction that is substantially normal to axis A1. In someembodiments, reflector 802 includes one or more surface layers (e.g.,metals, dielectrics, etc.) for improving its reflectivity for lightsignal 118.

Grating 804 is a vertical grating coupler formed in silicon waveguide116 and configured to receive light signal 118 from reflector 802 andredirect it along longitudinal axis A2 of the silicon waveguide.

As will be apparent to one skilled in the art, a vertical gratingcoupler typically has a range of angles at which light signal 118 can bereceived and successfully coupled into a waveguide. As a result,reflector 802 can be configured to redirect the light signal along anyangle within the acceptance range of grating 804.

In some embodiments, grating 804 is formed in passive waveguide 114 andreflector 802 is formed in silicon waveguide 116.

In some embodiments, silicon waveguide includes an output portcomprising a reflector configured to launch light signal 118 out of theplane of the silicon waveguide as a free-space signal.

FIGS. 9A-B depict schematic drawings of cross-sectional views ofalternative output ports in accordance with the present disclosure. Eachof ports 900 and 902 include reflector 904, which is formed in siliconwaveguide 116.

Reflector 904 is analogous to reflector 802 described above; however,reflector 904 is formed in silicon waveguide 116 to redirect lightsignal 118 as indicated.

Port 900 includes reflector 904 and facet 906. Facet 906 is formed insilicon waveguide 116 such that light signal 118 is launched into freespace and received by reflector 904. Reflector 904 is configured toredirect free-space light signal 118 away from handle substrate 202.

Port 902 includes reflector 904 is configured to launch light signal 118as a free-space signal directed toward substrate 908.

Substrate 908 is analogous to substrate 108 described above; however, itis preferable that substrate 908 comprise a material that issubstantially transparent and non-absorptive for the wavelengths oflight signal 118 so that the light signal can pass completely throughthe substrate with little or no attenuation.

While it is preferable in most applications to optically couple lightsignal 118 between OA device 110 and silicon waveguide 116, in someembodiments, the light signal is provided to, or received from, anexternal device or system without being coupled into the siliconwaveguide.

FIG. 10 depicts a schematic drawing of a cross-sectional view of analternative system in accordance with the present disclosure. System1000 is analogous to system 100; however, in system 1000, light signal118 is launched into free space directly from passive waveguide 114.System 1000 is disposed on substrate 1002 and includes facet 1004 andbulk reflector 1006.

Substrate 1002 is a bulk silicon substrate suitable for use in planarprocessing.

Facet 1004 is an end facet formed in passive waveguide 114 inconventional fashion (e.g., by etching, dicing, partial dicing, etc.).At facet 1004, light signal 118 exits the passive waveguide asfree-space signal 1008.

Reflector 1006 is analogous to reflector 804; however, reflector 1006 isa bulk reflector mounted on substrate 1002 such that it receivesfree-space signal 1008 from passive waveguide 114. In some embodiments,reflector 1006 is formed in a region of coupling layer 208 outside ofthe area of passive waveguide 114.

In some embodiments, it is preferable to precisely locate a bulk opticalelement to receive free-space signal 1008 directly from passivewaveguide 114.

FIG. 11 depicts a schematic drawing of a cross-sectional view of anotheralternative system in accordance with the present disclosure. System1100 is analogous to system 1000; however, system 1100 includes bulkoptical element 1102 and alignment feature 1104. Element 1102 isprecisely located on substrate 1002 by alignment feature 1104 such thatit receives light signal 118 from passive waveguide 114.

In the depicted example, element 1102 is an optical fiber; however,element 1102 can include a wide range of device and systems withoutdeparting from the scope of the present disclosure. Devices and systemssuitable for use in element 1102 include, without limitation, opticalfibers, PICs, photodetectors, light sources (edge-emitting lasers,vertical-cavity surface-emitting lasers (VCSELs), light-emitting diodes,integrated-optics systems, planar-lightwave circuits (PLCs), and thelike.

Alignment feature 1104 is a channel etched in substrate 1002 viaconventional methods (e.g., reactive-ion etching,crystallographic-dependent etching, ion milling, laser-assisted etching,etc.) such that its depth aligns the core of element 1102 with passivewaveguide 114. In some embodiments, alignment feature 1104 includes atleast one projection disposed on the top surface of substrate 1002 (orsilicon device layer 206 in embodiments where system 1100 is disposed onan SOI, such as substrate 108), where the projection is configured toconstrain element 1102 in at least one dimension.

It should be noted that, although the illustrative embodiment is anintegrated-optics-based, silicon-waveguide-coupled laser wherein lightis generated in an optically active device and propagates to a waveguideport, embodiments in accordance with the present disclosure includesystems wherein a light signal is coupled into a waveguide port andconveyed to an OA device. Furthermore, in some embodiments, no siliconwaveguide is included and port 120 is located in passive waveguide 114.Furthermore, in some embodiments, more than one passive waveguide and/orsilicon waveguide is included.

Including a narrow taper and/or sharp point in one or both waveguidelayers of a coupler enables efficient transfer of light between them,can reduce the length of a coupler, and/or mitigate reflections;however, their fabrication using conventional lithography and dryetching can be challenging.

Alternative methods for forming tapers includes the formation of arelatively blunt feature using lithography and dry etching, followed bya wet etch (often a crystallographic-dependent etch) to refine the bluntfeature into the desired shape. Unfortunately, wet etches can benotoriously difficult to control and extremely precise exposure timesare often required to avoid under or over etching the feature.Furthermore, material and/or layer properties (e.g., material density,layer thickness, residual stress, etc.) can vary across the surface of awafer giving rise to variations in etch results.

It is an aspect of the present disclosure that the inclusion ofetch-stop layers at key points in a layer structure can significantlyimprove fabrication of waveguide couplers, as well as other criticalfeatures.

FIG. 12 depicts a schematic drawing of a top view of yet anotheralternative embodiment of an integrated-optics system in accordance withthe present disclosure. System 1200 is analogous to system 100 andincludes active region 1202, transition region 1204, and output-couplingregion 1206, which are disposed on substrate 108.

FIGS. 13A-E depict schematic drawings of sectional views of system 1200through lines f-f through j-j, respectively.

FIG. 14 depicts operations of a method suitable for forming system 1200in accordance with the present disclosure. Method 1400 begins withoperation 1401, wherein device layer 206 is patterned to define slabregion 1208 and silicon waveguide 1222.

At operation 1402, optical element 1212 is formed in device layer 206within active region 1202.

Optical element 1212 is typically formed by etching features into slabregion 1208 and filling them with material having a refractive indexthat is different than that of silicon. In the depicted example, opticalelement 1212 is a diffraction grating whose gaps are regions of silicondioxide and whose grating elements are the unetched silicon between thegaps. In some embodiments, features of optical element 1212 are formedduring operation 1401 during the patterning of device layer 206 todefine the slab region and silicon waveguide. In some embodimentsoptical element 1212 can be configured to provide vertical confinementof the optical mode from below.

At operation 1403, coupling waveguide 1214, active-material stack 210,and etch-stop layers 1304-1 through 1304-3 are added to substrate 108,as described above and with respect to operation 302 of method 300. Asdiscussed above, in some embodiments, these layers are added tosubstrate 108 via heteroepitaxial growth. In some embodiments, theselayers are added to the substrate via a hybrid-bonding approach such asthat described above. In the depicted example, active-material stack 210includes gain layer 212, which functions as active waveguide 1306.

Coupling waveguide 1214 includes n-contact layer 1302, etch stop layer1304-1, and coupling layer portion 208B, which are collectivelyanalogous to coupling waveguide 208 described above.

N-contact layer 1302 is analogous to sub-layer 208A of coupling layer208. In the depicted example, N-contact layer 1302 is a layer of galliumarsenide that is doped to reduce its refractive index and increase itsconductivity.

Each of etch-stop layers 1304-1 through 1304-3 is a thin layer ofmaterial suitable for functioning as an etch stop during the patterningof the material residing on it. In the depicted example, etch-stop layer1304-1 is a very thin layer of aluminum gallium arsenide (AlGaAs). Aswill be apparent to one skilled in the art, however, myriad materialchoices exist for etch-stop layers suitable for use with the constituentlayers of compound semiconductor devices. For example, AlGaAs, indiumaluminum phosphide (InAlP), indium gallium aluminum phosphide, and thelike, are all suitable stop-etch materials suitable for use whileetching GaAs, GaAs is a suitable stop-etch material for use whileetching AlGaAs, and so on.

At operation 1404, etch-stop layer 1304-3 is patterned to define thelateral dimensions of gain section 1210 and taper 126-1. Gain section1210 is analogous to gain section 124 described above. The shape of gainsection 1210 determines the shape of OA device 1216. In someembodiments, the shape of taper 126-1 is based on at least one crystalplane of the materials of active-material stack 210.

In some embodiments, an etch stop layer is included on top of gain layer212. In some embodiments, an etch stop layer is included below gainlayer 212 such that the gain layer is disposed on the etch-stop layer.

At operation 1405, active-material stack 210 is etched in a suitableetchant down to etch-stop layer 1304-2. During operation 1405, etch-stoplayer 1304-3 protects the top surface of active-material stack 210,enabling its shape to be defined with very high fidelity with respect tothe mask used to pattern etch-stop layer 1304-3. Preferably, the edgesof the etched structure are protected during operation 1405 to mitigateetchant attack on the sidewalls of active-material stack 210 as the etchproceeds.

In some embodiments, the etchant used in operation 1405 is acrystallographic-dependent etch that stops on specific crystal planes inactive-material stack 210 to define the sidewalls of taper 126-1. Insome embodiments, a dry etch is used in operation 1405. in someembodiments, a wet etch is used in operation 1405.

In some embodiments, at least one layer of active-material stack 210and/or coupling waveguide 1214 includes a graded material compositionthat enables a specific vertical etch profile to be realized whenpatterning the layer(s).

The patterning of active-material stack 210 realizes active region 1202as being analogous to active region 102; however, active region 1202includes optical element 1212 located in device layer 206 beneath OAdevice 1216 to collectively define a DFB laser.

It is another aspect of the present disclosure that the portion ofactive-material stack located in the region of coupler 1218-1 (i.e., itstapered portion) functions as an optical amplifier for the lightgenerated within the DFB laser cavity.

It should be noted that, although the depicted example includes adiffraction grating optically coupled with active-material stack 210 togive rise to a DFB laser, any practical grating structure can be usedwithin a gain cavity or as a mirror that defines at least one end of again cavity of any suitable laser (e.g., distributed Bragg reflector(DBR) laser, coupled-cavity (CC) laser, etc.) without departing from thescope of the present disclosure. Examples of alternative gratingssuitable for use in accordance with the present disclosure include,without limitation, windowed sampled gratings (WSG), binarysuperstructure gratings (BSG), binary superimposed gratings, and thelike, some of which are described in U.S. Non-Provisional Pat.Application No. 17/465,403, which is incorporated herein by reference.

Furthermore, in some embodiments, optical element 1212 is formed bypatterning device layer 206 or active-material stack 210, rather than bydefining it in device layer 206, as also described in detail in U.S.Non-Provisional Pat. Application No. 17/465,403.

Still further, although the depicted example includes a Bragg gratingwithin active region 1202, it will be clear to one skilled in the art,after reading this Specification, how to specify, make, and usealternative embodiments in which an optical element other than a Bragggrating is included in active region 1202. Alternative optical elementssuitable for use in accordance with the present disclosure include,without limitation, photonic crystals, optically resonant cavities,holograms, and the like.

At operation 1406, the lateral dimensions of coupling waveguide 1214 aredefined in each of active region 1202 and transition region 1204.Typically, coupling waveguide 1214 is patterned in a multi-step processthat begins by patterning etch-stop layer 1304-2 such that it has thedesired lateral dimensions of the ridge portion of coupling waveguide1214. Once etch-stop layer 1304-2 is patterned, the material ofsub-layer 208B is etched in a suitable etchant that stops on etch-stoplayer 1304-1. The exposed etch-stop layer 1304-1 is then patterned todefine the lateral dimensions of the slab portion of coupling waveguide1214, followed by etching the material of n-contact layer 1302.

In some embodiments, one or both of sub-layer 208B and n-contact layer1302 is etched using a crystallographic-dependent etch that stops onspecific crystal planes in their material. In some embodiments, a dryetch is used to pattern one or both of sub-layer 208B and n-contactlayer 1302. In some embodiments, a wet etch is used to pattern one orboth of sub-layer 208B and n-contact layer 1302.

It should be noted that, in the prior art, the compound-semiconductorfeatures of integrated-optics systems have been subject to significantdamage because these features extend past the lateral extent ofunderlying silicon features, creating channels in which wet-etchchemicals can flow. This has led to undercutting of thecompound-semiconductor features, deposition of debris, and the like.

It is an aspect of the present disclosure, however, that, in someembodiments, no compound semiconductor feature extends past the lateraldimensions of slab region 1208 or silicon waveguide 1222 at any point.As a result, undercutting of these features is avoided entirely. In someembodiments, the perimeter of the compound-semiconductor material issupported by silicon material of silicon device layer 206 (e.g., regionsof slab region 1208 or silicon waveguide 1222). In some embodiments, atleast a portion of the compound-semiconductor material is supported bysilicon dioxide (or another suitable material) that has been formed tobackfill an etched region of device layer 206.

Such an arrangement of materials is enabled by the fact that, inaccordance with the present disclosure, silicon waveguide 1222 onlyguides light in output-coupling region 1206, which includes coupler1218-2 and silicon waveguide 1222.

At operation 1407 contacts 122 n and 122 p are formed as discussedabove.

As noted above, the active-waveguide layers of active-material stack 210and optical element 1212 collectively define OE device 1216 as a DFBlaser. In gain section 1210, optical mode 216 is contiguous and existentin active waveguide 1306, coupling waveguide 1214, and optical element1212. In addition, coupling waveguide 1214 transitions to passivewaveguide 1220 along the length of coupler 1218-1 (i.e., couplingwaveguide 1214 and passive waveguide 1220 are contiguous sections ofcoupling layer 208 that abut where active region 1202 and transitionregion 1204 meet).

Slab region 1208 has two distinct portions, a first portion on whichcoupling waveguide 1214 is disposed, and a second portion on which nocompound semiconductor material is disposed. In the depicted example,device layer 208 is configured such that it does not laterally confinelight (i.e., it does not confine light along directions parallel to theplane of substrate 108) in the second portion. In other words, lateralconfinement of an optical mode existent in both slab region 1208 andcoupling waveguide 1214 only arises from the lateral confinementprovided by the coupling waveguide (and active-material stack 210 whenit is also present).

At coupler 1218-1, widths w1(x) and w2(x) become narrower along thex-direction, which drives optical mode 216 out of active waveguide 1306such that the optical mode is distributed across coupling waveguide 1214and slab region 1208.

In transition region 1204, optical mode 216 propagates toward port 120as a contiguous optical mode that extends over coupling waveguide 1214and slab region 1208.

At coupler 1218-2, widths w4(x) and w5(x) become narrower along thex-direction, which drives optical mode 216 into silicon waveguide 1222such that the optical mode resides substantially completely in siliconwaveguide 1222 by the time it reaches output-coupling region 1206.

It is another aspect of the present disclosure that any of the etchsteps included in method 1400 can be performed in a two-step process inwhich a substantially “coarse” mask pattern is first formed (e.g., in anetch-stop layer) and then transferred into the layer to be patternedusing a first etch. Once the coarse shape of the patterned layer isformed, a second etch comprising a crystallographic dependent etchant isused to refine the shape of the patterned layer such that at least onesidewall of the patterned layer is defined by a crystal plane of itsmaterial. Such a two-step formation process enables definition ofsharper and/or narrower features than can be formed using conventionaletching methods known in the prior art.

In some embodiments, at least one of active-material stack 210, couplingwaveguide 1214, and silicon waveguide 1222 is configured such thatoptical mode 216 includes multiple optical-mode portions that arediscontinuous, as discussed in the parent application.

As noted above, in order to avoid trapping etchant chemicals and residuebeneath the compound-semiconductor structures, at each point along thex-direction, silicon material extends past the edges of passivewaveguide 1220. In addition, it is yet another aspect of the presentdisclosure that, within coupler 1218-2, silicon waveguide 1222 isconfigured such that, as optical mode 216 transitions from passivewaveguide 1220 fully into the silicon waveguide via coupler 1218-2,higher-order modes (e.g., optical modes excited due to misalignment ofthe compound-semiconductor features to the silicon features, excited byfinite taper-tip widths, emission from OE 1212, etc.) are suppressed inlight signal 118.

In the depicted example, radiation of higher-order modes out of lightsignal 118 is enabled by the inclusion of mode suppressors 1224 incoupler 1218-2. Mode suppressors 1224 are silicon “slab-waveguide”regions having width w7, which are optically coupled with siliconwaveguide 1222 such that the optical energy of the higher-order modescan couple into them.

Mode suppressors 1224 are features etched into device layer 206 to anintermediate depth d1 along the length of coupler 1218-2. After a lengthsufficient to enable a desired level of suppression, the width, w7, ofsuppressors 1224 reduces to zero at a point past the end of coupler1218-2 by etching the device layer to depth d2, which is deeper than d1.In the depicted example, depth d2 is equal to the full thickness ofdevice layer 206, thereby terminating suppressors 1224. In someembodiments, depth d2 is less than the complete thickness of the devicelayer. By virtue of the inclusion of suppressors 1224, coupling of thehigher-order modes into output port 120 via silicon waveguide 1222 ismitigated.

Typically, the width of suppressors 1224 reduces to zero width after theend of coupler 1218-2 in order to enable optical routing in a higherconfinement waveguide. However, in some embodiments, some portion of thewidth of the suppressors remains beyond the extent of coupler 1218-2. Insome embodiments, suppressors 1224 are configured to have additionalfunctionality, such as high-confinement structures that enable tighterwaveguide bends, higher waveguide density, or to form other photonicelements (e.g., ring resonators, power splitters, etc.).

It is to be understood that the disclosure teaches only examples ofembodiment in accordance with the present disclosures and that manyvariations of these embodiments can easily be devised by those skilledin the art after reading this disclosure and that the scope of thepresent invention is to be determined by the following claims.

1. An integrated-optics system disposed on a substrate that defines afirst plane, the system comprising: (1) an optically active device thatis selectively located in a first region of the substrate, wherein theoptically active device includes: (a) a coupling waveguide that includesa first layer disposed on a contact layer, the first layer comprising afirst compound semiconductor and the contact layer comprising a secondcompound semiconductor, wherein the coupling waveguide at leastpartially supports a first optical mode of a light signal; and (b) anactive-material stack comprising a third compound semiconductor, theactive-material stack being disposed on the coupling waveguide; whereinthe active-material stack and the coupling waveguide collectively definea composite waveguide that at least partially supports the first opticalmode; and (2) a silicon layer that includes a silicon waveguide that isselectively located in a second region of the substrate, the siliconwaveguide being configured to at least partially support the firstoptical mode; and wherein the coupling waveguide extends from the firstregion to a second taper located in the second region, the second taperbeing configured to optically couple the first optical mode from thecomposite waveguide into the silicon waveguide.
 2. The system of claim 1wherein the coupling waveguide further includes an etch-stop layer thatis located between the first layer and the contact layer, and whereinthe etch stop layer comprises a first material that has a first etchrate in a first etch, and wherein the first compound semiconductor has asecond etch rate in the first etch, and further wherein the first etchrate is slower than the second etch rate.
 3. The system of claim 2wherein the second taper includes at least one sidewall that is definedby a crystal plane of the first compound semiconductor.
 4. The system ofclaim 1 further including (3) an etch-stop layer, wherein the couplingwaveguide is disposed on the etch-stop layer, and wherein the etch stoplayer comprises a first material that has a first etch rate in a firstetch, and wherein the first compound semiconductor has a second etchrate in the first etch and the second compound semiconductor has a thirdetch rate in the first etch, and further wherein the first etch rate isslower than each of the second and third etch rates.
 5. The system ofclaim 4 wherein the coupling waveguide includes at least one sidewallthat is defined by a crystal plane of the first compound semiconductor.6. The system of claim 1 further including (3) an etch-stop layer,wherein the active-material stack includes a gain layer that is indirect physical contact with the etch-stop layer.
 7. The system of claim6 wherein the gain layer comprises a fourth compound semiconductor andincludes at least one sidewall that is defined by a crystal plane of thefourth compound semiconductor.
 8. The system of claim 1 furtherincluding (3) an etch-stop layer, wherein the active-material stackincludes a cladding layer and a gain layer that includes at least onequantum element that is selected from the group consisting of a quantumdot, a quantum well, a quantum dash, and a quantum wire, and wherein theetch-stop layer is between the cladding layer and the gain layer.
 9. Thesystem of claim 8 wherein the gain layer comprises a fourth compoundsemiconductor and includes at least one sidewall that is defined by acrystal plane of the fourth compound semiconductor.
 10. The system ofclaim 1 wherein the silicon layer is configured such that it has a firstportion that defines the silicon waveguide, a second portion on whichthe coupling waveguide is disposed, and a third portion on which thecoupling waveguide is not disposed, wherein the second portion and thecoupling waveguide collectively provide light confinement in a firstdirection that is parallel to the first plane, and wherein the thirdportion confines light in only a second direction that is orthogonal tothe first plane.
 11. The system of claim 1 wherein the couplingwaveguide has a bottom surface, and wherein the entirety of the bottomsurface is in physical contact with silicon or silicon dioxide.
 12. Thesystem of claim 1 wherein the second taper and the silicon waveguidecollectively define a first coupler, and wherein the silicon waveguideis configured to enable radiation of a second optical mode having ahigher order than the first optical mode away from the first coupler.13. The system of claim 1 further comprising (3) an optical element thatis optically coupled with the active-material stack.
 14. The system ofclaim 13 wherein the optical element and active-material stackcollectively define a laser selected from the group consisting of adistributed feedback (DFB) laser, a distributed Bragg reflector (DBR)laser, and a coupled-cavity laser.
 15. The system of claim 13 whereinthe optical element is configured as a mirror for partially defining alaser cavity in the active-material stack. 16-33. (canceled)
 34. Anintegrated-optics system disposed on a substrate, the system having afirst region, a second region, and a third region, the system including:a coupling waveguide that resides in the second region and extends fromthe first region to the third region, the coupling waveguide including afirst layer disposed on a contact layer, the first layer comprising afirst compound semiconductor and the contact layer comprising a secondcompound semiconductor, wherein the coupling waveguide at leastpartially supports a first optical mode of a light signal; an opticallyactive device that includes an active-material stack comprising a thirdcompound semiconductor, the active-material stack being disposed on thecoupling waveguide to collectively define a composite waveguide that atleast partially supports the first optical mode, wherein the opticallyactive device is located only in the first region; and a silicon layercomprising a silicon waveguide in the third region, the siliconwaveguide being configured to at least partially support the opticalmode, wherein the silicon waveguide is optically coupled with theactive-material stack via the coupling waveguide.
 35. The system ofclaim 34 wherein the coupling waveguide further includes an etch-stoplayer comprising a first material that has a first etch rate in a firstetch, the first compound semiconductor having a second etch rate in thefirst etch that is faster than the first etch rate, and wherein theetch-stop layer is located either (i) between the first layer and thecontact layer or (ii) beneath the contact layer such that the couplingwaveguide is disposed on the etch-stop layer.
 36. The system of claim 34wherein the silicon layer is configured such that it has a first portionthat defines the silicon waveguide, a second portion on which thecoupling waveguide is disposed, and a third portion on which thecoupling waveguide is not disposed, wherein the second portion and thecoupling waveguide collectively provide light confinement in a firstdirection that is parallel to the first plane, and wherein the thirdportion confines light in only a second direction that is orthogonal tothe first plane.
 37. An integrated-optics system disposed on a firstsubstrate, the system including: a coupling waveguide that at leastpartially supports a first optical mode of a light signal, the couplingwaveguide including a first layer disposed on a contact layer, the firstlayer comprising a first compound semiconductor and the contact layercomprising a second compound semiconductor, wherein the couplingwaveguide is located in a first region of the substrate and a secondregion of the substrate that abuts the first region; an optically activedevice that includes an active-material stack comprising a thirdcompound semiconductor, the active-material stack being disposed on thecoupling waveguide to collectively define a composite waveguide that atleast partially supports the first optical mode, wherein the opticallyactive device is located only in the first region; and an output portthat is located outside the first region; wherein the output port isoptically coupled with the active-material stack via the couplingwaveguide.
 38. The system of claim 37 wherein the coupling waveguideincludes the output port.
 39. The system of claim 37 further comprisinga silicon layer comprising a silicon waveguide in a third region of thesubstrate, the silicon waveguide being configured to at least partiallysupport the optical mode, wherein the silicon waveguide includes theoutput port, and wherein the output port is optically coupled with theactive-material stack via the coupling waveguide and the siliconwaveguide.
 40. The system of claim 37 wherein the coupling waveguidefurther includes an etch-stop layer comprising a first material that hasa first etch rate in a first etch, the first compound semiconductorhaving a second etch rate in the first etch that is faster than thefirst etch rate, and wherein the etch-stop layer is located either (i)between the first layer and the contact layer or (ii) beneath thecontact layer such that the coupling waveguide is disposed on theetch-stop layer.